All rotary machinery having active magnetic bearings requires an emergency device commonly referred to as an emergency bearing, to act in the event of one of said magnetic bearings being overloaded or in the event of the electrical or electronic control circuit failing, or indeed in the event of stoppage or of suspension overload. This emergency device provides mechanical redundancy and needs to be defined and dimensioned correctly so as to guarantee fully that the machine will not be damaged, and that the equipment can recover immediately once the overload or servo-control failure has disappeared. Such emergency bearings are generally dry-lubricated rolling bearings, but other types of dry bearing could be used. They present clearance relative to the rotor and in normal operation they do not rotate. The clearance is generally equal to half the airgap, but it can be smaller if necessary. Such bearings as described, for example, in French patent document 2 613 791, are ball bearings or, smooth rings commonly mounted relative to the stator on dampers and presenting radial clearance.
Rotors mounted on magnetic bearings often present nominal speeds of rotation that are very high. Under such circumstances, in the event of the rotor landing on the emergency bearings due to a failure of servo-control or of power supply, the rotor presents whirling movements at its speed of rotation with eccentricity which is then defined by the clearance of the emergency bearing. This whirling motion is also referred to as radial whiplash. Under such circumstances, a very high degree of unbalance can arise leading to destruction of the bearings or to deformation of the rotor.
It is then particularly important for the operation of the damper is guaranteed, where its function is firstly to prevent rotor whirl starting in the event of contact with the emergency bearing, and secondly to limit the frequency of this radial whiplash motion to a frequency that is low enough to avoid damaging the ball bearings or the smooth rings used. Typically, this frequency lies in the range one-fourth to one-third the frequency corresponding to the nominal speed of rotation of the machine. So long as freedom of movement in the radial direction is ensured for the bearings, radial flattening of the assembly strains the metal tape of the damper or any equivalent viscoelastic means, thereby satisfying the above condition.
In certain turbomachines, the axial load in operation can be considerable. So long as the axial magnetic bearing is operating correctly, nothing disturbs proper operation of the emergency bearing for radial failure. However, in the event of high axial loading that is not controlled by the magnetic bearing, the emergency bearing device must also be capable of capable of withstanding the high axial loading.
FIG. 1 shows the effect of an axial load 100 transmitted from the rotor to the stator through a ball of a prior art emergency bearing. Since contact area is quite small, it is difficult to organize a coefficient of friction that is very small or to avoid Hertz pressure that is too high. In the event of high axial loading, these two restrictions can lead to the emergency bearing becoming jammed radially in its housing instead of straining the damper device 130. Under such circumstances, the whirl or radial whiplash is synchronous with the speed of rotation and therefore leads to radial loads that are considerable and sometimes destructive of ball bearings (or smooth ring bearings if they are used instead of ball bearings), even if the machine comes to rest quickly, in general after a few seconds due to the braking that comes from the operating process or from the driving machine. This situation can become harmful for the equipment and consequently for magnetic bearing technology.